In order to access a desired track on an optical disk such as a CD, a DVD, or the like, in which storage and reproduction tracks are formed in a spiral shape or a circular shape, an optical disk reproduction apparatus moves an optical pickup module in a radial direction and a focus direction of a lens, to perform reading and writing. The optical pickup module is generally provided with 4 photodetectors, and based on photodetector detection signals respectively obtained from the 4 photodetectors, access to tracks and reading and writing of data is performed.
For example, as shown in Patent Document 1, in an optical disk reproduction apparatus, when a reproduced signal is generated according to a DVD-RAM format, in a case where an RF signal of a header part written to a track is generated, generation takes place based on a detection signal from one side, divided in a track direction, of a 4-division photodetector, or a detection signal from the other side. The RF signal of a data portion written to a track is generated based on a signal sum of 4 detection signals.
Furthermore, in reproduction of an optical disk of a conventional CD/CD-ROM format, similar to reproduction of the DVD-RAM format, generation is normally based on a signal sum of detection signals of the 4 photodetectors.    [Patent Document 1]    JP Patent Kokai Publication No. JP-P2007-257677A